1. Field of the Invention
The invention relates to a technique for controlling the optical properties of material surfaces. More particularly, the invention relates to an improvement in the transmission of light into and through an insulating material employing etching of a surface bombarded by high energy radiation.
2. Background of the Invention
It is preferable in many instances to reduce light reflection from the surfaces of insulating materials. For example, solar thermal absorber glazings can obviously be made more efficient in operation by reducing light reflection; see, for example, Journal of Vacuum Science and Technology, Vol. 12, pages 1023-1031 (1975). Other optical components for which reduction in light reflection improves operating efficiency are also apparent.
One way to reduce light reflection is by adding layers on the surface of the components which operate by the interference of light reflected from the different interfaces; see, e.g., B.O. Seraphin, "Chemical Vapor Deposition Research of Fabrication of Solar Energy Converters", NSF/RANN Report SE/GI-3671x/PR/74/4, 1974. It is also known to produce interference layers by etching the surface of the component in such a way that its index of refraction is uniformly lowered in a substantially discrete layer (or two) at the surface; see, e.g., "Optical Coatings for Flat Plate Solar Collectors", ERDA Final Report COO-2625-75/1.
A common drawback of the foregoing approaches is due to the discreteness of the antireflection layer (or layers), the reflectivity is lowered only for narrow bands separated by maxima in the reflectivity. In addition, the wavelengths of the minima are sensitive to the orientation of the surface with respect to the incident radiation. In order to be useful in a specific application then, the thickness of the layer (or layers) must be precisely controlled.
It is also well-known to reduce reflectivity with a smooth gradient in the index of refraction at the surface. That is, the index of refraction varies from the bulk value (which may be between about 1.5 and 5.0) to some value close to 1 in a smooth, continuous manner as the surface is approached from the inside. In one embodiment, this is known as the "moth's eye effect"; see, for example, Endeavor, Vol. 26, pp. 79-84 (1967). The moth's eye effect takes its name from a naturally occurring phenomenon--an array of protuberances of specific height and spacing--observed in the cornea of nocturnal insects. The array of protuberances which produces this effect has been reproduced using a photoresist and an interference pattern generated by a laser; see, e.g., Nature, Vol. 244, pp. 281-282 (1973). However, there was no net gain in transmission because of the absorption of the photoresist. A somewhat similar surface has been obtained, as reported in Journal of the Optical Society of America, Vol. 66, pages 515-519 ( 1976), by etching a phase-separated glass. Both surfaces show the characteristics of graded surfaces in that the surface reflectivity is reduced for wavelengths shorter than some transition wavelength and never go through maxima for shorter wavelengths. However, both of the foregoing techniques require special materials.